Preheat the oven to 350F degrees. Line a muffin pan with muffin papers.
In a large bowl sift together the cake flour, cocoa and salt. Whisk gently.
In a separate large bowl beat the butter until fluffy (about 2 minutes). Then beat in the sugar until creamy (about 1-2 more minutes).
Beat in the egg and vanilla extract.
In a liquid measuring cup whisk together the buttermilk and red food coloring.
Beat the dry ingredients (flour mixture) into the butter mixture about ⅓ at a time, alternating with about ⅓ of the buttermilk mixture. Turn off the mixer and scrape down the sides of the bowl as needed.
In a small bowl or cup dissolve the baking soda into the vinegar. Quickly beat it into the cupcake batter.
Spoon the batter into the prepared muffin papers, filling each about ⅔ full. Bake in the preheated oven for 15-18 minutes, or until an inserted toothpick comes out clean. Allow the cupcakes to cool in the pan for 5-10 minutes, before removing from the pan and continuing to cool on a wire rack.
Cream Cheese Frosting
In a large bowl using an electric mixer on medium speed, beat the butter until fluffy.
Mix in the cream cheese until evenly combined. Then add in the vanilla and salt.
In a separate bowl sift together the powdered sugar and cornstarch (if using).
Beat the powdered sugar into the butter and cream cheese mixture about 1 cup at a time, starting with the mixer on low and turning it up to medium speed as the sugar incorporates.
Frost the cooled cupcakes using a piping bag and tip (I used a 1M) or a knife.
Notes
*If you don't have cake flour, measure 1 ⅓ cups all-purpose flour. Remove 3 tablespoons, and add in 3 tablespoons of cornstarch. Sift the mixture together 3 times.**If you don't have buttermilk, add 1 teaspoon of white vinegar or lemon juice to ½ cup milk (I recommend 1%, 2% or whole milk). Let it sit for 5 minutes to sour before using.***Store cupcakes in an airtight container at room temperature for up to 8 hours, or in the fridge for up to 3 days. Remove the cupcakes from the fridge 1 hour before enjoying.